S-adenosylhomocysteine hydrolyase plays a critical role in regulating AdoMet-dependent methylations in eukaryotic cells, since it is the only enzyme present for the removal of S-adenosylhomocysteine. The purified enzyme has been used to study its structure and catalytic properties. The enzyme from rat liver has been purified to homogeneity and antibodies directed against the purified enzyme have been produced in rabbits. The role of NAD and cAMP binding in regulating the catalytic activity has been studied, and a large number of analogos of adenosine and adenosylhomocysteine have been examined for their ability to function as inhibitors and/or substrates both in vitro and in vivo. The synthesis of analogs of adenosylhomocysteine by this enzyme in vivo may be used to form very potent and specific inhibitors of transmethylation reactions, and provide useful probes for studies on the role of specific methylation reactions in biological functions. These analogs have a wide range of biological activities, including antiviral activity against a number of RNA and DNA viruses, inhibition of chemotaxis in mouse macrophage cell lines, and stimulation of cell differentiation in myoblast and erythroid cell lines. The two most interesting compounds studied are 3-deazaadenosine and 3-deazaaristeromycin. Both are inhibitors of adenosylhomocysteine hydrolyase, but only 3-deazaadenosine is a substrate for the enzyme. 3-Deazaadenosine, but not 3-deazaaristeromycin inhibits chemotaxis by a mouse macrophage cell line. In vivo, several methylation reactions, including phospholipid methylation, protein lysine and arginine methylation, and protein carboxyl methylation are inhibited to the same extent by both compounds. However, 3-deazaadenosine specifically inhibits synthesis of a small number of proteins, and is a potent inhibitor of mRNA synthesis.